It is known to provide auxiliary fuel tanks which may be externally carried on an aircraft (sometimes referred to as “Drop Tanks”). Exterior tanks are typically connected to an aircraft fuel system by means of a two-part valve which comprises an aircraft valve attached to the aircraft and a complimentary valve on the tank. The valve must provide a connection between the external tank and the aircraft system and provide a leak-proof seal between the tank and the aircraft.
A fuel tank valve according to an existing design is shown in FIG. 1 (with a cross sectional profile shown in FIG. 1b). The valve 10 comprises a valve body 12 having a generally cylindrical profile so as to define a central bore through which fluid may flow. A telescopic seal member 14 is received within the valve body and extends the central bore beyond the external surface 24 of the valve body 12. The telescopic seal member 14 is biased towards an extended position by a spring 18. In use, the spring is intended to be in a compressed state, as the telescopic seal member 14 is at least partial pressed back into the valve body 12. Thus, the spring provides a sealing force at the seal surfaces. The proximal (i.e. internal) end of the telescopic seal member 14 is provided with a seat 22 on its external surface for receiving an O-ring to seal against the internal surface of the valve body 12. The distal (i.e. external) end of the telescopic seal member 14 is provided with a resilient portion in the form of a compressible elastomer seal 20 which, in use, is intended to form a sealing engagement with a planar seal surface of the aircraft. The telescopic member 14 is arranged such that it has a degree of lateral freedom within the valve body. Specifically, a clearance gap is maintained between the telescopic seal member 14 and both the valve body 12 (for example between the seal seat 22 and the internal surface of the valve body 12) and the hole 26 in the outer surface 24 of the valve body. Additionally, the telescopic seal member is provided with a tapered external profile which converges slightly towards its distal end. The telescopic seal member may for example be arranged such that it may tilt its longitudinal axis by up to 5 degrees. This freedom is intended to allow the seal to both align with the aircraft seal surface when fitted to the aircraft and to accommodate any relative dynamic movement between the tank and aircraft during maneuvers on the ground or in flight.